Resinous composition and heater plate mica insulation produced therewith



1, 1950 E. L SCHULMAN 2,49,16 RESINOUS COMPOSITION AND HEATER PLATE MICA INSULATION PRODUCED THEREWITH Cas for 01'! "Per Cent m Raflm m 2 E N l R W m m 4m WITNESSES:

Patented Jan. 17, 1950 RESINOUS COMPOSITION AND HEATER PLATE MICA INSULATION PRODUCED THEREWITH Earl L. Schulman, Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application December 30, 1947, Serial No. 794,612

4 Claims. (Cl. 1542.6)

This invention relates to bonded mica and to the composition of the bonding material used therein.

In the manufacture of mica sheets for heater plates for electrical appliances such as toasters and electric irons, where the sheet is composed of mica flakes bonded together with a resinous binder, sufiicient binder must be applied to the mica flakes to form a rigid coherent sheet, so that the sheet can be subsequently handled and fabricated. On the other hand, it is desirable to keep the amount of the resinous binder in the finished sheet as low as possible so that a minimum of smoke and pungent fumes will be given off when the binder is burned out of the sheet after it has been incorporated in an electrical appliance.

It has been found that when mica sheet is manufactured by an automatic machine using resinous binders such as shellac, or mixtures of shellac and copal gum, or alkyd resins, the finished sheet must contain a minimum of about 5% to 6% of the binder or the sheet will not hold together during fabrication. Sheets having this amount of binder frequently stick to the cloth pads used in pressing the sheets making it difficult to remove the pads from the mica. This excessive amount of binder also evolves large volumes of smoke and acrid fumes when it is burned out after the sheet has been incorporated into electrical apparatus.

The object of this invention is to provide a bonded mica sheet having a low percentage of resinous binder of a novel composition.

A further object of the invention is to provide a bonded mica sheet having a resinous binder composed of a mixture of shellac, castor oil and a petroleum hydrocarbon insoluble pine wood resin.

For a further understanding of the nature and objects of the invention, reference should be had to the following detailed description, taken in conjunction with the accompanying drawing, in which:

Figure l is a view in perspective of a sheet of mica embodying the features of the invention, and

Fig. 2 is a triaxial diagram showing the composition of the bonding material used in practicing the invention.

In accordance with this invention, it has been found that certain compositions of shellac, castor oil and a petroleum hydrocarbon insoluble pine wood resin will provide a more satisfactory bond in the manufacture of mica sheet particularly pine oil, is extracted with a coal tar hydrocarbon, such as benzene or toluene, and the extract is then freed of volatile constituents leaving a resinous residue consisting of a mixture of wood rosin and the resin used in the present compositions. Extraction of this residue with a petroleum hydrocarbon, such as gasoline, dissolves and removes the rosin. After separation of the l rosin which is high in abietic acid, a resinous residue remains which is low in abietic acid and which comprises the resin employed according to the present invention.

Alternatively the residue of the initial coal tar hydrocarbon extract may be treated with a mixture of a petroleum hydrocarbon and furfural and the two layers which form are separated. The petroleum hydrocarbon insoluble resin is further dissolved in the furfural from which it may be separated by evaporation of the furfural. Other known methods of isolating this resin may be employed and if desired,'it may be steamed or heat treated to remove volatile constituents.

This resinous material is characterized by a dark red brown color, cherry red in solution and by substantial insolubility, that is, or greater, in petroleum hydrocarbons, but it will vary somewhat in its specific characteristics, such as acid number, melting point, solubility in aromatic hydrocarbons, etc., depending upon the details of the extraction process utilized. However, it will meet the following specification: Substantially insoluble in petroleum hydrocarbons, substantially complete solubility in alcohol, a methoxy content from about 3 to about 7.5%, an acid number between about 80 and 110, and a drop melting point between about C. and C.

While the proportions of the three ingredients utilized in practicing the invention may be varied somewhat, such proportions should be within the limits illustrated by Fig. 2. The compositions enclosed by lines l2, 2-3, 3--t and ll have been found satisfactory for use in bonding mica in the manufacture of mica sheets by automatic machines. The area enclosed by these lines corresponds to a composition of between 65% and 90% by weight of shellac, between and 20% by weight of castor oil, and between 5% and 15% hydrocarbon insoluble pine wood resin. The bonding material is commonly applied to the mica as a 5% to solution in a volatile solvent and the mica sheet is thereafter treated to drive 011 the solvent and heat and pressure applied to cure the resinous binder. The volatile solvent may be any common organic solvent used in dissolving pure shellac, such as methyl, ethyl or propyl alcohol or ethyl acetate.

In a specific embodiment of the invention, 7 parts of a composition comprising 81% by weight of shellac, 6% by weight of castor oil and 13% by weight of a hydrocarbon insoluble pine wood resin is dissolved in 93 parts by weight of ethyl alcohol. After the resinous solution has been applied to the mica flakes during manufacture and the sheet formed therefrom subjected to heat to evaporate the solvent, the composition of the resulting binder corresponds to that defined by point 5 in Fig. 2, or 81% shellac, 6% castor oil and 13% of hydrocarbon insoluble pine wood resin.

Referring to Fig. 1, there is illustrated a mica sheet l0, embodying the features of the invention. The sheet I0 is composed of mica flakes l2, bonded together with from 2% to 4% by weight of the resinous binder herein described. The sheet i0 is cured by the application of heat at a temperature of from 140 C. to 200 C., and pressure, which polymerizes the resinous binder. It is quite rigid and can be easily punched to shape and wound with heating element wire.

Mica sheet produced in accordance with the invention has several advantages over sheet produced using conventional binders. It has been found possible to use as little as- 2% of resinous binder and yet provide a sheet with sufficient strength to enable it to be handled by automatic machines during manufacture and fabrication of heating elements such as electric iron elements. The mica sheets with the binder of this invention do not stick to the pads utilized in pressing the mica during the curing operation. In addition, much less smoke and other deleterious fumes are given ofl when the binder is to be burned out of the sheet after incorporation into electrical apparatus.

Since certain obvious changes could be made without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted asillustrative and not in a limiting sense.

I claim as my invention:

1. Built up mica sheet comprising, in combination, mica flakes and a binder in an amount of from 2% to 4% of the weight of the mica flakes applied thereto, the binder composed of from to by weight of shellac, 5% to 20% by weight of castor oil, and 5% to 15% by weight of pine wood resin substantially insoluble in petroleum hydrocarbons.

2. Built up mica sheet comprising, in combination, mica flakes and a binder in an amount of from 2% to 4% of the weight of the mica flakes applied thereto, the binder composed of 81% by weight of shellac, 6% by weight of castor oil, and 13% by weight of pine wood resin substantially insoluble in petroleum hydrocarbons.

3. A resinous material composed of from 65% to 90% by weight of shellac, 5% to 20% by weight of castor oil, and 5% to 15% by weight of pine wood resin substantially insoluble in petroleum hydrocarbons.

4. A resinous material composed of 81% by weight of shellac, 6% by weight of castor oil, and 13% by weight of pine wood resin substantially insoluble in petroleum hydrocarbons.

EARL L. SCHULMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,276,304 Hunter Mar. 17, 1942 2,307,587 Hill et al. Jan. 5, 1943 2,373,996 Borglin Apr. 17, 1945 

1. BUILT UP MICA SHEET COMPRISING, IN COMBINATION, MICA FLAKES AND A BINDER IN AN AMOUNT OF FROM 2% TO 4% OF THE WEIGHT OF THE MICA FLAKES APPLIED THERETO, THE BINDER COMPOSED OF FROM 65% TO 90% BY WEIGHT OF SHELLAC, 5% TO 20% BY WEIGHT OF CASTOR OIL, AND 5% AND 15% BY WEIGHT OF PINE WOOD RESIN SUBSTANTIALLY INSOLUBLE IN PETROLEUM HYDROCARBONS. 